There can be as much as a 10% variance in different lots of the same powder. My friends and I have loaded many rounds for different cartridges over the years and found there is a difference in powder by as much as 10%. For example: I load a 180gr TSX in front of 84.5grs of RL-22. Now before anybody panics please read the rest before responding. Yes, this load is 4.5grs over book. Now I just did not jump up and say ok I will load 4.5grs over book and put my face and possibly my life on the line! I worked up to this load. When I got close to max load for the book I began to go up a half grain at a time checking my pressures by measuring the base of my brass after I fired each round knowing I would stop when the base expanded .0005". As I worked up a half grain at a time there was little to no expansion and primers looked fine and no shinny spots on the base of my brass. I began at 83grs to get expansion and that let me know I was getting close. At 84.5 I had expanded .0004 and decided to stop. Now why did I stop and not go up until the base expanded to .0005? Well, I had already reached outstanding velocities of 3287fps on the average and excellent accuracy and saw no need to continue. This was a very cool lot of RL-22 and here are two targets shot with this load to illusrate why I stop and did not need to push any further. The first one is a group size of .334" and the second one is a group size of .1005"
Now to make my point, I tried a different lot of RL-22 and could only reach 81.3grs when the base of the brass had expanded .0004. This was a hotter lot of powder and did not have as good accracy as the other lot at the same pressures as seen in the following target which was a grooup size of .452"
I would advise anyone who is reloading to work up. Have you ever wondered why at times your favorite pet load when you bought a new lot of the same powder did not give you the same results. Well it is possible that your new lot is hotter or cooler than the last lot you used. Please do not use any of my load data stated above with out working up because your lot could be as much as 10% hotter than mine was. Now I know some will think this is just anecdotal experience. Please work up to max and start over with every lot of powder, just a suggestion but it might be worth consideration. My gunsmith said he has fixed as well as thrown away a rifles and pistols that were destroyed or ruined by hand loaders.